Testing, testing...

[Tony Williams]

I haven't done a similar drawing for 6.8mm and 7mm, but going by this one, it looks as if you may be right about a "green" 6.5mm bullet not being heavy enough. Thoughts?

I think you've summed up the problems. Penetration into the case isn't necessarily an issue if the case capacity is big enough, but getting a good BC with less dense materials is difficult in small calibres; the length/diameter ratio could become a problem with stabilising such bullets.